This invention relates to a reversible electrode comprising mainly organic compounds for use as an electrochemical element in batteries, electrochromic display devices, sensors, memories, and the like.
Since the discovery of conducting polyacetylene electrodes made by Shirakawa et al. in 1971, a practical application of using conduction polymers electrodes has been actively pursued. When conducting polymers are employed as electrode materials, a realization of such electrochemical devices as a light battery of a high energy density, an electrochromic display device of a large display area, a biochemical sensor using a microelectrode etc. can be expected.
However, the problem is that polyacetylene is not chemically stable against the moisture and the oxygen which exist in the air and it cannot be used practically as the electrode for electrochemical devices. In order to solve the aforementioned problem, an introduction of other conducting polymers of electron conjugated system has been studied with a resultant finding of such relatively stable polymers as polyaniline, polyprole, polyacene, polythiophene, etc. These polymers have been used for development of lithium secondary batteries.
Not only cations but also anions within electrolyte are involved with the electrode reaction of these polymer electrodes. Since the electrolyte is serving as a carrier of ions and at the same time is involved with a battery reaction, a certain quantity of electrolyte corresponding to battery capacities needs to be held within the battery. As a result, battery's energy density is reduced for the aforementioned quantity of electrolyte to 20-50 Wh/kg, about one half of that of an ordinary secondary battery such as a nickel cadmium battery, a lead-acid battery and the like.
As an approach to solving the aforementioned problem by use of organic materials having potentially a high energy density, use of organic disulfide compounds was proposed in a European patent, No. 415856. The foregoing compounds are expressed generally by a formula of X--S--R--S--(S--R--S).sub.n --S--R--S--X', wherein n is 0 or any integer from 1 and above, X and x' are a metal M, an alloy containing the metal M, a hydrogen atom or an organic terminal function group, R is a cyclic compound comprising a carbon atom combined with at least one sulfur atom S of thiol.
The S--S bond of this compound is broken down by an electrochemical reduction and a salt represented by R--S--M.sup.+ is formed by bonding with a cation M.sup.+ contained in the electrolyte. Also, this salt is characterized by returning to the original R--S--S--R by an electrochemical oxidation. In addition, in the aforementioned European patent is proposed a metal-sulfur secondary battery wherein a metal M for releasing or capturing cation (M.sup.+) is put together with an organic disulfide compound with a resultant energy density of at least 1000 Wh/Kg, considered comparable with or exceeding that of an ordinary secondary battery.
The fact that the oxidation reaction of organic disulfide compounds is promoted by the addition of a low molecule, flavin, which is an organic compound of nitrogen containing conjugated systems, is described in Journal of the American Chemical Society, Vol. 97, No. 11, pp. 3235-3238, (1975).
In other words, it is mentioned that the sulfur atoms contained in organic disulfide compounds and capable of disulfide bonding are bonded with the nitrogen atoms of organic compounds of nitrogen containing conjugated system with a resultant promotion of reaction.
However, only the reaction velocity is dealt with in the aforementioned paper and anything about the oxidation promotion effect caused by organic compounds of nitrogen containing conjugated system against organic disulfide compounds is not measured nor discussed from an electrochemical approach. Besides, nothing at all is mentioned in the foregoing literature about the fact that, by using organic disulfide compounds together with conducting polymers as the organic compounds of nitrogen containing conjugated system for promoting the oxidation/reduction reaction, electrodes having an excellent reversibility in organic solvents at room temperature, capable of handling large charging/discharging currents, can be produced.
Although the addition of conducting polymers to organic disulfide compound electrodes as a conducting material or a collector material simply like carbon, etc. is mentioned in the specification of the aforementioned European patent No. 415856, the fact that conducting polymers promote the oxidation/reduction reaction in electrolysis of organic disulfide compounds is not at all referred to.
Particularly, the fact that the conducting polymers, prepared by polymerization of a monomer having a structure wherein nitrogen atoms are conjugated with carbon atoms, interact with organic disulfide compounds serving as a catalyst is not touched upon.
As reported in an embodiment of the specification of the European patent No. 415856, a battery built by using [(C.sub.2 H.sub.5).sub.2 NCSS--].sub.2 for example shows small discharge/charge currents of 13 .mu.A/cm.sup.2 and 6.5 .mu.A/cm.sup.2 respectively in its charge/discharge performance.
According to a battery reaction theory, the electron mobility process in the electrochemical reaction of these materials is progressing very slowly and their usage is limited to the high temperature ranges of 100.degree.-200.degree. C.
It is difficult to drain a practically required current as large as at least 1 mA/cm.sup.2 for example at room temperature.